The purpose of the proposed research program is to gain more detailed insight into the mechanisms responsible for active sodium transport across epithelia and, in particular, mammalian small and large intestine. A theoretical equivalent electrical circuit model has been developed which should permit the analysis of intracellular electrical potentials in terms of the resistances and electromotive forces responsible for sodium transport across the mucosal and basolateral membranes of the epithelial cell. Using standard electrophysiologic techniques, these equivalent electrical analogs will be evaluated under controlled conditions and in the presence of pharmacologic, pathologic and humoral agents that influence active sodium transport. In addition, we have recently found that the calcium ionophore, A23187, abolishes active chloride absorption and elicits active chloride secretion by rabbit colon and simulates the effects of cyclic AMP on this tissue. The effects of cyclic AMP on calcium content and exchange of rabbit colon will be investigated.